In recent years, the number of vehicles having a navigation machine onboard has rapidly increased. The navigation machine holds a digital map data base and displays a traffic jam or an accident position on a map based on traffic jam information or accident information which is provided from a traffic information center, and furthermore, executes a path search by adding their information to conditions.
The database of the digital map is created by several companies, and the map data include an error because of a difference in a basic view and a digitization technique and the error differs depending on a digital map created by each company. For this reason, in the case in which an accident position is to be transmitted through traffic information, for example, there is a possibility that the accident position might be identified on the wrong road depending on the type of the digital map data base held in the onboard machine if longitude and latitude data on the position are presented alone.
In order to improve the accuracy of the information transmission, conventionally, a node number is defined to a node such as an intersection present in a road system and a link number is defined to a link representing a road between nodes, each intersection and road are stored corresponding to a node number and a link number in a digital map data base created by each company, a road is specified based on a link number in traffic information, and a point on a road is displayed by an expression method, for example, a distance from a head. However, the node number and the link number which are defined in the road system should be changed according to the new construction or alteration of a road. Moreover, if the node number or the link number is changed, digital map data created by each company are to be updated. For this reason, a method of transmitting the position information of a digital map by using a node number and a link number requires a great social cost for maintenance.
In order to improve such a respect, the inventors of the invention have proposed the following method in JP-A-11-214068 and JP-A-11-242166.
In this method, when transmitting the position of a road on which an event such as a traffic jam or an accident occurs, the information providing side transmits, to the receiving side, “road shape data” comprising a coordinate string having (1) a node in which the road shape of a road section having a predetermined length including the event position is arranged on the road (2) and an interpolation point (the vertex of a polygonal line approximating the curved line of the road, which will be referred to as a “node” including the interpolation point if there is no notice in this specification) and (3) “event position data” indicative of an event position based on a relative position in the road section represented by the road shape data; and the side receiving this information carries out map matching by using the road shape data, specifies a road section on a self-digital map, and specifies an event generation position in the road section by using the event position data.
Moreover, the inventors of the invention have also proposed a method in which a procedure for the map matching is executed efficiently. This method employs a sequential matching technique, and the receiving side calculates the coordinates of an event position by using the received road shape data and event position data and adds the event position as a node in the node string of the road shape data. Then, the map matching is executed in order from a node on the start edge of the node string and a point which best matched with a node indicative of the event position is specified as the event position on the road of a self-digital map.
In the case in which the position information of the digital map is to be transmitted by these methods, there is an important problem in that matching precision on the receiving side is to be enhanced. In the sequential matching method, particularly, when the start point of the map matching is wrong, the error tends to propagate to the subsequent map matching so that mismatching is apt to be caused. Moreover, there is a problem in that the mismatching is easily generated in an intersection having a small intersecting angle.
The invention solves these problems and has an object to provide a method of transmitting the position information of a digital map which can enhance matching precision on the receiving side.